3-COLOR AND 1+1-RESONANCE IONIZATION MASS-SPECTROMETRY OF ZIRCONIUM SPUTTERED FROM REFRACTORY CARBIDES

Although resonance ionization mass spectrometry (RIMS) has proved a powerful tool in trace analyses of pure elemental materials, much less attention has been given to refractory multicomponent materials of interest in many geochemical applications. As preparation for analyses of minor amounts of Zr in refractory carbide interstellar grains, we have detected zirconium atoms sputtered from Zr metal and a ZrC powder, using both 1 + 1 and three-color RIMS. The two spectroscopic schemes were quantitatively evaluated for studies of chemically complex samples via (1) matrix sensitivity measurements, in which the Zr resonance signals obtained from ZrC and Zr metal under identical experimental conditions were compared; (2) measurement of the resonant enhancement of Zr sputtered from ZrC, relative to (isobarically interfering) Mo and Ti-2 sputtered from Mo2C and TiC samples, respectively; and (3) for the three-color study, measurement of the saturation curves for the individual Zr isotopes for each of the laser wavelengths employed. No Ti-2 signal was observed above the analog noise level for either scheme. The three-color ionization is easily saturated and yields a Zr/Mo selectivity of 3600 +/- 1400. An odd-even Zr isotope effect was clearly evident in a comparison of the measured off- and on-resonance Zr-91 abundance. The results here should prove useful in quantitative RIMS analyses of compound refractory matrices and in the assessment of the potential for RIMS in studies of interstellar carbide grains.